The 19 hijackers who carried out the worst act of terror ever to occur on U.S. soil worked with little outside help as a single, integrated group composed of identifiable leaders and shadowy foot soldiers who prepared for their final day in a tight choreography over 18 months.

An examination of public records and dozens of interviews shatters the image of the conspiracy that coalesced immediately after the Sept. 11 attacks.

Based on early, flawed information from federal investigators, initial accounts depicted an operation that was carried out by four compartmentalized cells of terrorists. And because investigations and neighbors were confused by similar or falsified Arabic names, reports emerged that the cells included as many as 10 pilots, who -- with wives and children -- had blended seamlessly into suburban America.

In fact, it now seems clear that only a single hijacker aboard each of the four commandeered aircraft knew how to fly a plane. Just two of the other hijackers -- both linked to terrorist Osama bin Laden -- had briefly taken flight lessons.

These six men apparently formed the conspiracy's leadership. Records and interviews show that this core group, often separated by thousands of miles, remained in the United States the longest and left behind the most visible tracks that, in retrospect, can be seen as highly synchronized preparations.

Some of the leaders were educated, worldly and so intimately connected that three of the four suspected pilots had roomed together in Germany, where they attended the Technical University of Hamburg. Sophisticated as they were, the leaders were clumsy enough in their English and manners that they repeatedly provoked notice and annoyance, if not outright suspicion, while they were in the United States.

Helping these leaders was a cadre of 13 Saudi Arabian men, most of them younger and less educated, many from their country's poorest regions. These young Saudis left faint appearances in U.S. public records and seem for the most part to have arrived only in recent months.

Leader or follower, none of the hijackers brought wives or children with them. And contrary to early reports, none of the pilots had worked for Saudi Arabian Airlines.

For the leaders and followers alike, a maze of connections -- including overlapping addresses -- exists among hijackers who ended up on different flights.

The synchronization of their preparations is evident in the most basic ingredients of their plot. Seven of the hijackers obtained Florida driver's licenses within a 15-day span in early summer. Thirteen purchased airline tickets for their final flights within five days in late August. And over the course of the summer, a dozen -- who ultimately ended up spread among the four flights -- moved through South Florida apartments.

The plot revolved around mundane, perfectly legal details of everyday life: tourist visas, driver's licenses, apartment leases, Internet connections, airline tickets, mail boxes and rental cars. The records left by the hijackers as they carried out those ordinary acts reveal the footprints of the conspiracy. They detail who did what and with whom, and they reveal that the hijackers were divided into two distinct classes.

"There are two groups on each plane: You've got the brains, who are the pilots and the leaders, and then you have the muscle coming in later on," said a senior government official. "They were the ones who held the passengers at bay."

This newer portrait of the conspiracy may yet evolve. The FBI investigation into the plot is preliminary, and the conspiracy's precise nature probably will not be understood for years. Only a fraction of what has been learned about the conspirators by federal investigators is publicly known. Telephone records and airline manifests, for example, would be disclosed only in secret before a grand jury or in a courtroom.

But from the information that is available at the moment, certain patterns can be gleaned that render a fuller picture of the conspirators.

In particular, an analysis of the hijackers' visible trails gives greater clarity to the role of Mohamed Atta, the 33-year-old Egyptian lawyer's son already identified by a government official as the "axel" of the plot. He traveled the most, listed the most addresses, took the most practice flights and had the greatest interaction with other conspirators. Atta and two of the other suspected pilots -- Marwan Al-Shehhi and Ziad Samir Jarrah -- belonged to a radical Islamic student group in Hamburg that investigators believe may have been a birthplace of the plot.

More broadly, both the leaders and the followers can be seen to have often deployed in pairs. They came together for crucial tasks, such as to get new government identification cards that would ease their passage onto the planes.

The hijackers' behavior reveals certain incongruities. They were Islamic fundamentalists who nevertheless indulged in Western culture, from fast food to hard liquor. One spent $4,500 on a single airline ticket, yet they haggled over bar tabs, car rental fees and apartment security deposits just days before they would die.

The most basic incongruity, though, is this: The preparations of the 19 hijackers were imperfect. Some were kicked out of pilot schools. Some had to pay cash for their plane tickets after their credit cards were rejected. Two were late for the Boston flight that would be the first to slam into the World Trade Center. But inexact as it was, their plot succeeded in claiming more than 6,000 lives.

The Advance Guard In November 1999, two Saudi Arabian men moved into a ground-floor apartment at the Parkwood Apartments, a town house complex near a busy commercial strip in San Diego. Khalid Almihdhar and Nawaf Alhazmi struck their neighbors as odd. They had no furniture but often carried briefcases and seemed to be on their cell phones a lot.

Two months later, investigators believe, Almihdhar and Alhazmi traveled to Malaysia, where they met with bin Laden operatives who were later linked to the bombing of the destroyer USS Cole.

By May 2000, they arrived at Sorbi's Flying Club, a small school 20 miles north of San Diego that trains about four dozen pilots a year, and announced that they wanted to learn to fly Boeing airliners.

Almihdhar and Alhazmi were part of the advance guard.

Their flight lessons began within weeks of the day two of the other leaders, Atta and Al-Shehhi, a 23-year-old native of the United Arab Emirates, enrolled in a six-month course at Huffman Aviation, a flying school in Venice, Fla.

A continent apart, the four men displayed uncanny parallels. According to former neighbors, landlords and flight instructors, the California team and the Florida team almost always left their apartments as a pair. Few people recall ever seeing any of them alone.

Within each pair, one man assumed a more genial, communicative role, while the other was quieter, brooding. In California, Alhazmi is remembered as more outgoing. In Florida, waitresses and others consistently recall Al-Shehhi as friendlier than Atta -- a dour, arrogant man whose English seemed atrocious at times, but suddenly could be smooth when he needed a car or hotel room.

These four men traveled often: Al-Shehhi to Morocco and Amsterdam, Atta twice to Spain.

Neither team took pains to be furtive. Although Atta occasionally used aliases, all four men gave their real names when they registered for flight lessons or bought airline tickets -- a violation of a "terrorist's manual" written for bin Laden's network.

Almihdhar and Alhazmi, in particular, were readily visible within the local Muslim community. They mingled at the Islamic Center of San Diego. It was at the center that they bought the blue Corolla they would ultimately drive across the country and park at Dulles International Airport on Sept. 11.

Even as they sought to blend into the United States well enough to complete their tasks, the pairs of men were imperfect chameleons. At times, they were overeager. They were hindered by faulty English. They were, on occasion, aggressive, even boorish.

Rick Garza, Sorbi's chief flight instructor at the time, sat Almihdhar and Alhazmi down after a half-dozen ground lessons and two flights. "This is not going to work out," he told them.

Their English was terrible, but Garza was more disturbed by a certain overzealousness. Even though "they had no idea what they were doing," the instructor said, they insisted on learning to fly multi-engine planes, at one point offering him extra money if he would teach them.

In Florida, Atta strived to adapt to U.S. styles, shedding the flowing beard and tunic he had favored in Germany for a clean-cut look. But both he and Al-Shehhi, while more successful than the San Diego pair at acquiring pilots' skills and licenses, could be similarly off-putting. At Huffman, Atta appropriated the seat cushion of a fellow student while he flew in the school's Piper Cherokee Warrior.

Infuriated, the student, Anne Greaves, tried to wrest the cushion from Atta's grasp. "Marwan lunged, putting his arm quickly between Atta and myself, to protect him in a way," Greaves said. "I remember thinking, 'What on Earth could they be frightened of?' "

Doughnuts by the Boxful If the behavior of the first four was conspicuously unpleasant, they nevertheless were clearly more adept than the young Saudi men who came in a second wave.

One of these men, who moved early last summer into a shabby apartment building in Paterson, N.J., once had to ask a neighbor how to screw in a light bulb.

Among the first to arrive were Hamza Alghamdi, 20, and Mohand Alshehri, 23, who in January rented a post office box in Delray Beach, Fla.

Most of the second group of conspirators were from poor families. A few had enough education to give them skills that would prove handy. Alshehri, who graduated from a religious high school and dropped out of Imam Muhammed bin Saud University, was facile enough with computers that he could use the Internet at a Delray Beach public library.

But these younger men seemed to settle under the wings of a leader for such basic needs as finding a place to live. Last winter, Hani Hanjour, another pilot, did the talking when he rented the Paterson apartment with another young man, even though Hanjour's own English was poor. In June, Al-Shehhi, by then a licensed pilot who had been in Florida for at least a year, helped Hamza Alghamdi shop for an apartment, according to the real estate agent who worked with them.

Unlike the first wave, who focused on the mentally rigorous work of pilot training, the second wave of young men put time into strengthening their bodies. In Florida and Maryland, they paid cash to train with weights in gyms.

In ways that were curiously out of sync with Islamic orthodoxy, these young men seemed to revel in their brief taste of American life. They wore shorts and T-shirts. Last month, Majed Moqed, 22, another hijacker on American Airlines Flight 77, which hit the Pentagon, stopped into a Beltsville store that rents adult videos. After scanning the titles, he did not rent any, but he returned at least once.

Some of the hijackers who passed through New Jersey during the summer developed the habit of buying doughnuts by the boxful and meals from a Chinese carryout. Others frequently stopped by a bar at night for Salem or Parliament cigarettes, Heineken or Budweiser beer.

A Blur of Motion New Jersey served as one hub for the conspirators in the days and nights of summer. South Florida served as the other. Soon, the early pairs gave way to larger, interlocking groups.

The apartment that Al-Shehhi had helped Hamza Alghamdi to find also became the home of Saeed Alghamdi and Ahmed Ibrahim A. Al Haznawi.

On Aug. 2, at least five -- and possibly seven -- of the hijackers went to a Virginia Department of Motor Vehicles office in Arlington, where they allegedly met a local man who fraudulently helped them obtain identification cards they could flash at airport counters.

The men who got the IDs that day later would fan out to three of the four hijacked planes, illustrating the conspiracy's interwoven nature. The scheme is striking for a second reason: It shows the amount of calculation behind the plot. The men who got the Virginia cards included those who would board the flight at nearby Dulles. The only others who took part in the scam were the two hijackers on other planes who had not obtained a driver's license in Florida since last spring.

Such close coordination, visible all along, is particularly evident as the conspirators purchased their tickets and moved into their final positions before the attacks. The last weeks of August and first days of September appear in retrospect as a blur of motion, as hijackers left apartments, returned rental cars and realigned to join the men with whom they would board their planes.

Mysteries That Linger As more of the conspiracy becomes understood, government sources now say that the investigation so far suggests the 19 had "no major help" in the United States. Sources say that the conspirators were funded with $500,000 from overseas and that the terrorist mission was planned and launched several years ago in Germany, with crucial support in Britain, the United Arab Emirates and Afghanistan.

Of the more than 480 people detained during the last few weeks, a few have drawn particular attention.

Zacarias Moussaoui was detained Aug. 17 after he caused a scene at a flight simulator in Minnesota, where he worried his instructors by baldly saying he wanted to learn how to fly jets but not to land them.

Two Indian men who had gotten off an airplane on Sept. 11 were arrested on a train in Fort Worth the next day. Accounts differ on what led to the arrests, but the men were discovered with $5,000 in cash, hair dye and box-cutter knives similar to ones used by the hijackers to take control of the planes.

Early last week, Mohammed Abdi, a Somalian working as a security guard in the District, was detained after authorities found his phone number written on a map left behind in the blue Corolla by several of the hijackers in a Dulles parking lot. And Friday, Lotfi Raissi, an Algerian pilot who had lived in Arizona, was accused in Britain of training four of the hijackers.

In recent days, the investigation has intensified in Germany as well, where authorities are seeking people who roomed with the hijackers from Hamburg or had other ties to them.

Of all the mysteries that linger, a central one surrounds the man believed to be the fourth hijacker pilot: Hanjour. Unlike the other three suspected pilots -- Atta, Al-Shehhi and Jarrah, who trained in Europe -- there is no evidence that Hanjour was radicalized in Islamic circles within Germany. Unlike the other pair of leaders -- Almihdhar and Nawaf Alhazmi, who have been linked to bin Laden's network and settled together in San Diego -- Hanjour did not train to fly with a partner.

Of all the 19, Hanjour's roots in the United States seem deepest. The first trace of him in this country dates to 1990, when he appeared at the University of Arizona in Tucson for an eight-week English course. Exactly a decade later, he received a student visa by applying for another English course, this time in Oakland, Calif. He entered the country but never showed up in class.

In his elusiveness, in his long acquaintance with America, Hanjour is the only hijacker who fits the profile of what investigators call a "sleeper," a terrorist who lives inconspicuously in a country for years before committing his violent act.

It is clear that Hanjour knew the San Diego leadership team. They were in the city together and, by some accounts, were roommates for a time. By last spring, he was on the East Coast, helping the younger group in New Jersey. What is less evident is his exact role in the conspiracy. Was he dispatched early to prepare the path? Was he taken into the plot as a pilot after the pair in San Diego proved so inept?

Certainly, Hanjour's own piloting skills were shaky. He took lessons at a Scottsdale, Ariz., flight school four years ago, but eventually was asked to leave by instructors who said his skills were poor and his manner difficult. Just a month ago, instructors at Freeway Airport in Bowie flew with him and deemed him unfit to rent a plane by himself.

But on the morning of Sept. 11, as Flight 77 veered off its course to Los Angeles and streaked toward Washington and the Pentagon, Hanjour is thought to have been the one who executed what a top aviation source called "a nice, coordinated turn."